This study was undertaken to determine the identity of the causative agents of the filariases reported from northwestern tetraonids (Fowle, 1946; Babero, 1953), the incidence of filarial infections in the Tetraonidae of British Columbia, and the means by which these birds acquire their infections. More than 400 wild gallinaceous birds of 10 species and over 200 adult non-gallinaceous birds from 27 families were examined. Adults of the following 4 species of filariae (Onchocercidae: Splendidofilariinae) were collected from the Tetraonidae of B.C. and/or Alaska and are described herein: 1) Skriabinocta flexivaqinalis (Jones, 1961) n. comb.; 2) Splendidofilaria pectoralis new species; 3) Splendidofilaria "species A"; and 4) Splendidof il aria papil locerca (Lubimov, 1946;) Anderson and Chabaud, 1959. Descriptions are provided also for the microfilariae of S. flexivaqinalis. S. pectoralis and S. papillocerca, and for Microfilaria laqopodis (Haaland, 1928),Brinkmann, 1950, and Microfilaria "species B" from tetraonid hosts. Recent systems of filarioid classification are evaluated and some modifications proposed, including revival of the genus Skriabinocta Chertkova, 1946 to comprise S. petrowi Chertkova, 1946, S. flexivaginalis. S. chitwoodae (Anderson, 1961) n. comb,, S. striatospicula (Hibler, 1964) n. comb., and provisionally S. lienalis (Orloff, 1947) n. comb. On Vancouver Island, Microfilaria sp. B is hyperenzootic in blue grouse, with a significantly higher prevalence in adult males than in adult hens or in yearlings or chicks. Mf. sp. B is enzootic in Vancouver Island ruffed grouse. S. flexivaginalis is sporadic in British Columbia ruffed grouse. Splendidofilaria pectoralis is sporadic to enzootic in tetraonids from central B. C. and Alaska. Mf. lagopodis is.enzootic in willow ptarmigan of northern B. C. Microfilariae closely resembling Mf. flexivaginalis occasionally parasitize non-gallinaceous birds of Vancouver Island, but Mf. sp. B and S. pectoralis seem to be restricted totetraonid hosts. The larvae of S. flexivaginalis develop in the thoracic muscles of the ceratopogonid, Culicoides unicolor (Coq.) group. Those of Mf. sp. B develop in abdominal fat bodies of the simuliids, Simulium aureum Fries and Cnephia minus (D. and S.); the former being considerably more efficient as an intermediate host. Development of Mf. sp. B fails at temperatures which do not range above 62°F. Larval development is described and on the basis of larval characters some features of the adults of Mf. sp. B are predicted. Knowledge acquired on the biology of the tetraonid hosts, the incidence of the filariases, and the seasonal abundance of the vectors and the effects of various factors on their feeding activities, is integrated in a discussion of the epizootiology of Skri abinoct a flexivaqinalis and Microfilaria sp. B. / Science, Faculty of / Zoology, Department of / Graduate